Copper Electrodeposition Kinetics Measured by Species

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Copper Electrodeposition Kinetics Measured by
Alternating Current Voltammetry and the Role of Ferrous
Species
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Citation
Wagner, Mary-Elizabeth, Rodrigo Valenzuela, Tomas Vargas,
Melanie Colet-Lagrille, and Antoine Allanore. “Copper
Electrodeposition Kinetics Measured by Alternating Current
Voltammetry and the Role of Ferrous Species.” J. Electrochem.
Soc. 163, no. 2 (November 5, 2015): D17–D23.
As Published
http://dx.doi.org/10.1149/2.0121602jes
Publisher
Electrochemical Society
Version
Final published version
Accessed
Wed May 25 18:13:38 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/101747
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Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
D17
Copper Electrodeposition Kinetics Measured by Alternating
Current Voltammetry and the Role of Ferrous Species
Mary-Elizabeth Wagner,a Rodrigo Valenzuela,b Tomás Vargas,b,c Melanie Colet-Lagrille,b,c
and Antoine Allanorea,∗,z
a Department
of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139-4307, USA
de Electroquı́mica, Departamento de Ingenierı́a Quı́mica y Biotecnologı́a, Universidad de Chile,
Santiago, Región Metropolitana, Chile
c Advanced Mining Technology Center (AMTC), Universidad de Chile, Santiago, Región Metropolitana, Chile
b Laboratorio
Impurities and additives play a key role in copper electrodeposition, in particular in upstream processes such as electrowinning or
electrorefining. One common impurity is iron, mostly present as iron species Fe(II) in highly concentrated sulfuric acid solutions and
in a cathodic environment. Herein, the kinetics of copper electrodeposition from such solutions have been investigated using a copper
rotating disk electrode and alternating current voltammetry (ACV). For a concentration of proton of 1.84 M and a concentration
of Fe(II) ions of 0.054 M, the deposition kinetics are slow enough to separately observe the two electron transfer steps involved in
copper reduction: an observation unique to ACV. The results suggest that Fe(II) ions affect the electrodeposition kinetic by slowing
down reaction kinetics, in particular slowing the second electron transfer reaction.
© The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0121602jes] All rights reserved.
Manuscript submitted July 8, 2015; revised manuscript received October 19, 2015. Published November 5, 2015.
Advancing the understanding of copper electrodeposition kinetics in a concentrated sulfuric acid electrolyte is beneficial at multiple
stages of copper life cycle. At the extraction and purification stage,
copper electrowinning and anode electrorefining are essential steps.
In both cases, the metal deposited on the cathode needs to be periodically recovered, requiring the deposit to remain structurally stable
throughout the duration of metal growth. In downstream copper applications, such as electronics manufacture, the presence of voids or the
lack of sufficient nuclei coalescence in electrodeposits can ultimately
compromise the integrity of products, e.g. computer chips. This last
issue has recently been driving research to improve deposit quality,1–3
but less work has been published on the challenges faced in upstream
copper production.
Various factors influencing copper electrodeposition have been
reported in conditions simulating the low pH, high copper concentration electrolytes common in industrial conditions.4–6 In upstream steps
such as electrowinning or electrorefining, solution additives play an
important, though only marginally understood, role in the final deposit
quality. Additives encompass both inevitable impurities inherited from
the raw minerals and carried along with copper during the smelting
process,7 as well as impurities purposely added to improve the deposit properties.8 The outcome on deposit quality varies widely,6–10
with additives exhibiting either a positive or negative effect on copper growth. Of particular interest in this work, Fe(II) ions have been
reported to have a positive effect,3,6 possibly due to slowing down the
reduction kinetics of copper.9 Because upstream copper production
involves deposition on copper cathodes, the present study is limited
to copper deposition on a copper substrate. Thus, our study is not one
of copper nucleation during electrodeposition, but rather speaks about
electrodeposition kinetics during growth.
Electrodeposition is a complex process drawing on multi-physics,
and the reduction process for the deposition of copper written as:
Cu 2+ + 2e− → Cu
[1]
actually involves many interrelated phenomena, such as mass transport, charge transfer, adatom diffusion, incorporation into growth sites,
and crystallization, each governed by their own rates. The situation
is further complicated for copper electrodeposition in concentrated
electrolyte, where a four-step mechanism, commonly referred to as
∗ Electrochemical Society Active Member.
z
E-mail: [email protected]
an (C)ECE reaction, is cited:11
Chemical r eaction (C)
−x H O
2
Cu 2+ (H2 O)6 ⇐⇒
Cu 2+ (H2 O)6−x
[2]
Electr on trans f er (E)
e−
Cu 2+ (H2 O)6−x ⇔ Cu + (H2 O)6−x
[3]
Chemical r eaction (C)
−y H2 O
⇐⇒ Cu + (H2 O)6−x−y
+
Cu (H2 O) 6−x −z H O,H +
2
⇐⇒
Cu + (H2 O)5−x−z O H −
Electr on trans f er (E)
Cu + (H2 O)6−x−y
Cu + (H2 O)5−x−z O H −
[4]
e−
⇔ Cu (0)
[5]
Such a mechanism involves a chemical reaction (C), in this case the
dehydration of the water molecule shell surrounding Cu(II); an electron transfer (E), the reduction of Cu(II) to Cu(I); a second chemical
reaction (C), the hydrolysis or dehydration of Cu(I); and a second
electron transfer step (E), the reduction of Cu(I) to Cu metal.
Of the two electron transfer steps involved in copper reduction, the
first (reaction 3) is reported as rate-limiting.12,13 Under certain conditions, such as near neutral pH electrolytes, copper electroreduction
signals only exhibit one electron transfer event.14
Subsequent researchers have built upon this mechanism,15,16 taking into account the intermediate chemical reaction steps related to
dehydration (deaquation of solvation shells, reaction 2 and 411,17–18 ).
Reaction 5 needs particular attention since it is the step leading to
actual metal deposition, and has been reported to occur via copper (I)
ion adsorption prior to its reduction.19 Certain aspects of copper reduction kinetics remain highly contested, especially when comparing
electrolytes of various pH,14 partially due to difficulty in discerning
the second electron transfer (reaction 5). The ability to make such
comparisons is essential in order to identify the role of additives, e.g.
Fe(II) ions.
The present study therefore aims at identifying a characteristic
electrochemical signal relevant to each of the electron-transfer reactions, in order to better understand how the chemical conditions
can ultimately affect the mechanism of copper adatom production
(reaction 5).
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D18
Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
Table I. Experimental conditions investigated. Redox potentials are calculated using standard redox data available in Ref. 31, assuming unit
activity for all species except the metal ions, for which activity is taken to be equal to concentration.
Solution
1a
1b
1c
2a
2b
2c
3
4
H2 SO4 (M)
Cu(II) (M)
Fe(II) (M)
Temperature (K)
Expected E Cu2+ /Cu (V/SHE)
Expected E Fe2+ /Fe (V/SHE)
AC amplitude (mV)
1.840
0.630
298
0.334
−
160
1.080
0.630
298
0.334
−
160
0.890
0.630
298
0.334
−
160
1.840
0.630
0.054
298
0.334
−0.515
160
1.080
0.630
0.054
298
0.334
−0.515
160
0.890
0.630
0.054
298
0.334
−0.515
160
1.840
0.010
273
0.286
−
80
1.840
0.010
0.054
273
0.286
−0.509
80
AC frequency (Hz)
7.75
7.75
7.75
7.75
7.75
7.75
7.75
7.75
Very few studies report the use of alternating current methods, including alternating current voltammetry (ACV), for copper
electrodeposition.16 The advantages of ACV methods were put forward by Smith,20 and more recent works by Bond have popularized
the technique.21 In ACV measurements, an AC waveform, such as a
sine wave, is superimposed onto a linear DC potential sweep. The
current response can then be converted into the frequency domain
via a Fast Fourier Transform (Supplementary Figure S1). In a plot of
power versus frequency, the DC component of the current response is
given by a sharp peak near zero frequency. Additional peaks, occurring at multiples of the AC wave’s frequency, called the harmonics,
represent the AC components of the current response.20 These individual peaks can be isolated and converted back into the time domain
to separately look at the current contribution of each harmonic.21 The
phenomena involved in copper electrodeposition can be separated into
faradaic, e.g., charge transfer, and non-faradaic components, e.g., double layer capacitance. Because faradaic processes respond to voltage
in a nonlinear fashion, and non-faradaic processes respond linearly, it
is possible to separate the two processes using AC techniques.1 The
linear terms, i.e. the non-faradaic processes caused by double layer
capacitance effects, will only be present in the DC current response
and the first fundamental harmonic AC response.21,23 Higher harmonics therefore allow examination of charge transfer kinetics without
interference from non-faradaic components. In addition, ACV techniques are useful in examining very fast kinetic reactions.21 ACV
therefore typically allows for electrode phenomena not easily seen in
DC voltammetry methods to be investigated.22–24 ACV seems then
appropriate for studying copper deposition because of the subsequent
electron transfer reactions involved, as well as a possible adsorption
step prior to metal deposition which is presumably very fast.
Few predictions are available as to the expected AC current response when adsorbed species are involved,25 and the authors were
not able to find theoretical or experimental studies on the AC voltammetry current response during copper electrodeposition.
This work aims at identifying and characterizing the multiple steps
involved in copper electrodeposition in sulfuric acid electrolyte, with
particular attention to the effect of Fe(II) ions on deposition kinetics.
Technically, and in order to exhibit evidence of the effects on kinetics,
ACV is used to investigate the electrochemical mechanism of copper
electrodeposition.
Procedures and Experimental Methods
Experimental setup.— A double-walled glass container was used
as the electrochemical cell hosting a three-electrode setup. The working electrode was a 6.58 mm diameter rotating copper disk (surface
area 0.34 cm2 ), polished to a mirror with 1 micron alumina. The electrode was rotated at 800 revolutions per minute in experiments seeking
controlled and forced mass transfer conditions. A coiled platinum wire
and a saturated calomel electrode (0.241 V vs. SHE at 298 K) were
used as counter and reference electrodes, respectively. All potentials
are hereafter reported with respect to the standard hydrogen electrode.
Solutions were prepared using reagent-grade sulfuric acid (95-98%
purity, Sigma-Aldrich), copper (II) sulfate pentahydrate (99% purity,
Alfa Aesar), and iron (II) sulfate pentahydrate (99+% purity, Alfa
Aesar), and deionized water (100% purity, RICCA Chemical). Powders were weighed first, to obtain the necessary amount of cupric and
ferrous ions, and then dissolved over a twenty-four hour period into a
solution with the desired molarity of sulfuric acid.
Experiments were either performed at ambient temperature, 298
± 5 K, or at water’s freezing point, 273 K. For the latter, temperature
was controlled by flowing ice water through the cell wall. Nitrogen
gas was bubbled through all electrolytes for at least 10 minutes prior to
electrochemical measurements. A summary of experimental solutions
and conditions is presented in Table I.
Each run first consisted in recording the electrochemical
impedance at the open-circuit potential to determine the uncompensated ohmic resistance. This value was obtained as the real component
of the impedance measured when the imaginary component crosses
the real axis on a Nyquist plot, typically for a frequency of around
100 kHz. The measured resistance was used to obtain the actual working electrode potential in post-experiment analysis, and all potential
data are reported with such correction.
Measurements with a rotating working electrode were conducted
for no more than 5 potential cycles with each freshly polished electrode. Non-rotating experiments were only run for one cycle. Except
where noted, all data presented hereafter are with rotation of the
cathode. The investigated potential range and the rotation rate were
optimized to limit the hydrogen evolution reaction, to limit anodic
dissolution of the copper electrode, and to ensure repeatability between cycles in both the DC component and higher AC harmonics.
The scan-rate was limited to a fairly fast value in order to limit copper
growth during each cycle. Only the third, fourth, and fifth cycles were
analyzed. At the end of the last cycle, the cell was dismantled and the
electrode was re-polished before the next run.
Methods of data collection and analysis.— The direct current
voltammograms were generated using a potentiostat (Reference 3000:
Gamry Instruments, USA). For alternating current measurements, a
sine wave created with a waveform generator (MOTU UltraLite-mk3
Hybrid: MOTU, USA) was superimposed onto the direct current ramp.
The current response was collected using a data acquisition system
(DT9837B: Data Translation, USA) at a 25 kHz acquisition frequency.
A custom designed code (ver. 5.5.2; Scilab Enterprises, France) was
used to analyze the results post-measurement. The complex current
response was analyzed using a Fast-Fourier Transform (FFT), which
presented a power spectrum with sharp peaks at multiples of the frequency of the AC sine wave (see Supplementary Figure S1). These
peaks represented the transformed DC, fundamental, and higher harmonic components, and were isolated and converted back into the
time domain using an inverse FFT. Thus, the DC and harmonic results
could be individually plotted versus the DC potential.21
In absence of a full ACV model for the electrodeposition mechanism of copper, which presumably involves adsorption, the measured
AC components were compared to a model drawn from experimental
results for a simpler case: a reversible, single electron transfer system
involving the ferri/ferrocyanide redox couple with 10 mM Fe(CN)6 3−
and 3 M KCl (see Supplementary Figure S2), with the overall reaction:
4−
−
Fe (C N )3−
6 + e → Fe (C N )6
[6]
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Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
D19
harmonic presents one sharp, symmetrical peak at the half wave potential, E1/2. The second harmonic is represented by the derivative of
the fundamental harmonic, with two symmetrical peaks and a trough
at E1/2 . The third harmonic is a derivative of this second harmonic,
with three peaks, the middle peak being located at E1/2 . This pattern
repeats for all higher harmonics.20 In this well-studied system, peak
heights scale proportionally with an increase in the electron transfer
coefficient, α, and with a faster rate constant k0 . A large α favors more
symmetrical peaks, in both height and shape.22 Four main features
are thus of interest in analyzing experimental results: peak heights on
both the anodic and cathodic sides of E1/2 , peak shape symmetry with
respect to E1/2 , overall peak height, and the location of E1/2 .
Results
Figure 1. Variation of the direct current during a potential sweep from
+0.400 V to −0.100 V in 1.84 M H2 SO4 and 0.63 M Cu(II) at ambient temperature, without (solution 1a, black, solid) and with 0.054 M Fe(II) (solution
2a, red, dotted). Scan rate is 50 mV.s−1 .
This reaction allows investigating a situation where the reaction-rate is
quasi-reversible, highly dependent on mass-transfer, and the kinetics
are relatively fast, features that are likely to be present in some steps
of copper electrodeposition. Under these conditions, the fundamental
Electrodeposition in concentrated solutions at 25◦ C (fast kinetic
conditions).— Figure 1 shows the DC signal measured during copper
electrodeposition in a solution with 0.63 M Cu(II) and 1.84 M H2 SO4 ,
concentrations representative of industrial practices. Multiple analyses
of the DC component do not allow for a clear distinction between the
current responses for solutions with and without 0.054 M Fe(II).
In ACV signals, as shown in Figure 2, the fundamental and
higher harmonic components reveal harmonic peaks characteristic
of a faradaic event, for all concentrations of H2 SO4 (solutions 1a, 1b,
1c, Table I). The peak center for this faradaic event shall henceforth
be denoted E1 . E1 ω , as observed for the fundamental harmonic, does
not align with E1 2ω , E1 3ω and E1 4ω as observed in higher harmonics.
The latter are positioned at 0.285 V/SHE across compositions and
harmonics, while E1 ω shifts toward a more cathodic potential with an
increasing concentration of H+ .
Figure 2. Variation of A) fundamental B) second C) third and D) fourth harmonic current with the DC potential for solutions 1a (blue, dotted, 1.84 M H2 SO4 ),
1b (red, dashed, 1.08 M H2 SO4 ) and 1c (black, solid, 0.89 M H2 SO4 ) at room temperature.
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D20
Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
Figure 3. Variation of the A) second and B) third harmonic current with potential for solutions 2a (blue, dotted, 1.84 M H2 SO4 ), 2b (red, dashed, 1.08 M H2 SO4 )
and 2c (black, solid, 0.89 M H2 SO4 ). C) Variation of the difference in the potential for the second harmonic current peaks (see A) and D) Variation of the peak
currents at E1 for the third harmonic current peaks (see B) with pH for solutions without (black, closed) and with 0.054 M Fe(II) (red, open).
In solutions with higher concentrations of H+ , the peak heights
become asymmetric with respect to E1 . In 1.84 M H2 SO4 , there is
a considerable peak height difference between the two peaks of the
second harmonic. In the third and fourth harmonic, the peaks that
occur on the anodic side of E1 are generally higher than their cathodic
counterparts, with the exception of the fourth harmonic in the extreme
case of 0.84 M H2 SO4 .
The peak shapes are also asymmetrical with respect to E1 . In
solutions with higher concentrations of H+ , the peaks at potentials
cathodic to E1 have a prominent shoulder around 0.150 V/SHE (see
inset in Figure 2C), which is absent from their anodic counterpart.
Figures 3A and 3B shows the second and third harmonics for solutions that contained Fe(II) ions at 0.054 M (solutions 2a, 2b and
2c, Table I). The measurements of the fundamental harmonic for such
solution (not shown) did not exhibit noticeable difference from the
one measured for a solution free of iron presented in Figure 2A. In
particular, they also exhibit a peak for E1 at around 0.043 V/SHE.
Other key features of the AC harmonics discussed above for solutions
without Fe(II) (Figure 2) were more pronounced in solutions containing Fe(II). This is illustrated by the prominence of the shoulder at
0.150 V/SHE in such solutions (see inset Figure 3B). Likewise, the
peak heights are more asymmetric than in solutions without Fe(II).
Such solutions also show wider harmonics peaks, with the anodic and
cathodic sides located further from each other (Figure 3C). In solutions with high concentrations of H+ , peak heights were lower when
Fe(II) ions were present (Figure 3D). As with peak distance, this effect
is less pronounced at a higher pH.
Electrodeposition in diluted copper solutions at 0◦ C (slow kinetic
conditions).— In order to investigate the possibility that the peak
shoulder located at 0.150 V/SHE in Figure 2 was a second faradaic
event, efforts were made to slow down electron transfer kinetics. The
next set of experiments diverged from previous conditions in favor
of a colder temperature (273 K) and a lower concentration of Cu(II).
These solutions (solutions 3 and 4, Table I) were prepared with 0.01
M Cu(II) and 1.84 M H2 SO4 , both with and without 0.054 M Fe(II).
Cyclic voltammetry experiments under these conditions (Figure 4)
still do not show a significant difference between solutions with and
without Fe(II).
Figure 5 is an overlay of the fundamental and second harmonic
from a solution at 273 K containing 0.01 M Cu(II), 1.84 M H2 SO4 and
0.054 M Fe(II). As anticipated for such conditions, lower AC currents
are measured, indicative that reaction rates were slower. Two separate
faradaic events can be observed, at 0.33 V/SHE and 0.04 V/SHE,
termed E1 and E2 , respectively. These two distinct fundamental harmonic peaks line up with two distinct second harmonic troughs. The
location of E1 ω at low temperature (Figure 5) is shifted anodically
compared to the room temperature data such that it is now aligned
with E1 observed for the higher harmonics.
Figure 6 shows the harmonics for the isolated first and second
faradaic events. With the exception of the E1 offset by around 25 mV,
no significant difference in peak height or shape are observed for
this event in solutions with or without Fe(II). In presence of Fe (II),
E2 (Figure 6, insets) is more cathodic and separated from E1, with
consistently lower harmonic peaks and more distinctive features than
in absence of Fe(II). Compared to the harmonics at E1 , and the results of the ACV model (see Appendix 2), the E2 peaks are atypical.
E2 3ω and E2 4ω are located at a more cathodic potential than E2 ω or
E2 2ω . This shift is similar to the one observed in Figure 2A, where
the location of E1 shifts between the fundamentals and the higher
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Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
Figure 4. Variation of the direct current during a potential sweep from
+0.400 V to −0.350 V in 1.84 M H2 SO4 and 0.01 M Cu(II) at 273 K,
without (solution 3, black, solid) and with 0.054 M Fe(II) (solution 4, red,
dotted). Scan rate is 50 mV.s−1 .
harmonics. The observed faradaic event at E2 is distorted towards more
anodic potentials due to overlap with the faradaic event at E1 , with
the strongest effect on the fundamental harmonic. Higher harmonics
appear less susceptible to such overlap, pertaining more strictly to the
D21
Figure 5. Overlaid fundamental (red, solid) and second (red, dashed) harmonic current response for solution 4 (0.01 M Cu(II), 0.054 M Fe(II), and
1.84 M H2 SO4 ) at 273 K.
charge transfer of interest, with peak positions stabilizing in the third
and fourth harmonic.
Although the peaks at E2 (Figures 6c and 6d) are strongly asymmetrical, they split at a consistent potential with alternating peaks (for
odd harmonics) and troughs (for even harmonics). This is a definitive
characteristic of a faradaic event in AC voltammetry, with the observed asymmetry being typical of an irreversible reaction.20,25 Such a
Figure 6. Variation of A) fundamental B) second C) third and D) fourth harmonic current with the DC potential for experiments in 1.84 M H2 SO4 and 0.01 M
Cu(II) at 273 K, without (solution 3, black, solid) and with 0.054 M Fe(II) (solution 4, red, dotted).
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D22
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Figure 7. Variation of A) second and B) third harmonic current with the DC potential for experiments in solution 4 (0.01 M Cu(II), and 0.054 M Fe(II), and
1.84 M H2 SO4 ) at 273 K, without (red, solid) and with rotation at 800 rpm (red, dotted).
current response shape has also been predicted for reactions involving
an adsorption step.25
In order to further investigate the electron transfers occurring at
E1 and E2 , measurements conducted in absence of rotation of the
working electrode are reported (Figure 7). The absence of rotation did
not affect the E1 signals, while peak currents at E2 increased and the
peak split in higher harmonics is more pronounced. Furthermore, the
harmonics at E2 in absence of rotation exhibit a similar shape to E1 ,
and are in more qualitative agreement with the ACV model.
Discussion
Copper electroreduction mechanism.— Depending on the experimental conditions highlighted earlier, one or two faradaic events may
be distinguished during copper electrodeposition. In industry-level
concentrations and at room temperature, DC measurements indicate
an inflection point at 0.25 V/SHE (Figure 1), a distinct event E1 also
observed in AC measurements in second and higher harmonics. At
lower temperatures and lower copper concentration, the location of E1
shifts anodically, as would be expected from a system with Nernstian
behavior. The location of E1 at a potential close to the standard electrode potential for Cu(II) reduction to Cu metal (Table I), and the lack
of any other thermodynamically possible faradaic reactions occurring
at that potential lead us to assign E1 to the following reaction:
e−
Cu 2+ ⇔ Cu +
[3]
This first step, according to the previous literature, is the slowest faradaic step in the overall copper electrodeposition reaction. DC
measurements in industrial concentrations also indicate a second inflection point at 0.05 V/SHE, E2 (Figure 1), an event not noticed as
clearly as E1 in AC measurements under the same conditions. Instead,
the second and third harmonic currents from E1 exhibit shoulders
overlapping with the current response at E2 . The shoulders are most
prominent in low pH solutions, which have been shown to be associated with a smaller α.11,26 The loss of peak symmetry could be
attributed to a decrease in α, or to the increasing prominence of the
event at E2 . Slowing overall reaction kinetics, either through decreasing pH, decreasing Cu(II) concentration, or decreasing temperature,
point to slower events at E1 and E2 , increasing the prominence of
the shoulder. Furthermore, at low pH, the harmonics for E1 are more
asymmetrical, again indicating overlap with the currents from E2 . This
overlap is consistent with ACV models for multiple electron transfer
steps.20
In the extreme case shown in Figures 5 and 6, the two reduction
steps at E1 and E2 are completely separated. In these cold solutions
with low concentrations of Cu(II), no shoulders on the cathodic side
of the reaction are observed. This confirms that the second event cor-
responds to a distinct electron transfer reaction, and that the shoulders
observed under industrial conditions are due to the overlap of E1 and
E2 currents. It is likely, therefore, that the event E2 is very fast compared to E1 , such that it is often masked by E1 current responses.
The event E2 exhibits atypical second and higher harmonic currents
exhibiting strongly asymmetrical shapes, pointing to an irreversible
reaction or a reaction involving an adsorbed species.20,25
It is possible that the hydrogen evolution reaction (HER) occurs
in parallel with the reaction at E2 , although studies of hydrogen evolution on copper electrodes report large overpotentials for HER.4,5,27
Furthermore, Fe(II) ions have been shown to be a catalyst for HER,27
which contradicts the decrease in currents observed for E2 in systems
in the presence of Fe(II) (Figure 6).
Results with a stationary working electrode show that the ACV
signal at E2 has a stronger dependence on rotation than at E1 , with
the event E2 appearing more distinct and better defined in absence
of rotation (Figure 7). For a mass-transfer controlled quasi-reversible
reaction, ACV signals have been shown to be insensitive to the masstransfer conditions for low-enough frequencies of excitation.28 This
result applies to our AC measurements for reaction 3 at E1 , which
involves soluble species and did not exhibit variation with or without
rotation. Nevertheless, it can be postulated that in absence of rotation,
the soluble product (Cu(I)) for reaction 3 stays in the vicinity of the
electrode and is less dispersed toward the bulk of the electrolyte. Such
Cu(I), confined to the electrode surface is then more available for
further reaction, explaining the enhanced AC signal at E2 .
The faradaic event E2 therefore appears as a second electron transfer step involving surface confined species, most likely following
reaction 5:
e−
Cu + ⇔ Cu
[5]
Previous authors have reported such a reaction occurring at similar
potentials, suggesting that the reaction involves Cu(I) as an adsorbed
species and is a very fast electron transfer reaction.18 Such a scheme
is in agreement with the mechanism presented in introduction, with
reaction 5 involving surface species, e.g. adsorbed species. That reaction 5 can be affected by the amount of Cu(I) supplied from reaction
3 and possibly the mass-transfer conditions is a feature that can be
evidenced thanks to the combination of ACV and RDE techniques.
The features of the fundamental harmonics for copper electrodeposition recorded in proton concentrated solutions remain anomalous,
in particular with regards to the shift of E1 ω from the Nernst reduction
potential for reaction 1. Such a shift is not observed in conditions
where reaction 5 can be isolated (see Figure 2 vs Figure 5 for example), suggesting that the measured reduction potential for reaction
1 in industrial-simulating concentrations represents a value between
the Nernst potentials for reactions 1 and 2. This is also consistent
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Journal of The Electrochemical Society, 163 (2) D17-D23 (2016)
with ACV models for a mechanism involving two successive electron
transfer steps.20
Influence of Ferrous Ions.— In very acidic solutions, the influence
of Fe(II) on the electron transfer kinetics is more predominant at low
pH. With Fe(II) present in the electrolyte, the harmonic peaks for reaction 3 remain strongly asymmetrical, even at a higher pH, as opposed
to measurements without Fe(II). This suggests that reaction 5, which
has harmonics better resolved in solutions with high concentration of
proton, is influenced by Fe(II), the presence of which enhances the
separation between reaction 3 and 5 (see Figures 3A and 3B).
Under conditions where the two reduction steps are distinguishable, no difference in harmonic shape for reaction 3 in solutions with
and without iron can be observed (Figure 6). Conversely, reaction 5
is highly influenced by the presence of Fe(II) (Figure 6, insets). Measurements in solutions containing Fe(II) reveal consistently decreased
current values; and peak shapes that more qualitatively fit the ACV
model than measurements without Fe(II).
When Fe(II) is present in the system, reaction 3 appears at more
anodic potentials, while reaction 5 occurs at more cathodic potentials.
Fe(II) ions therefore appear to be hindering reaction 5, thus enhancing
separation between the two electron transfers, while not influencing
reaction 3. Those results are a key step in isolating and studying
the two reactions independently, and in ultimately extracting kinetic
parameters for Cu(II) reduction.
Our results allow us to support the hypothesis that the electron
transfers in copper reduction obey the following mechanism:
e−
step 1
Cu 2+ ⇔ Cu +
step 2
+
Cu + ⇔ Cu ads
step 3
e−
+
0
⇔ Cu ads
Cu ads
The inconsistent trends with pH for step 3 (reaction 5), however, may
also indicate that Fe(II) affects the intermediate chemical reaction 4:
−y H O
2
Cu + (H2 O)6−x−y
Cu + (H2 O) 6−x ⇐⇒
in which case water solvation shell interactions between Fe(II) and
Cu(I) would have to be studied.
Lack of a fully developed model for ACV experiments on such
mechanism involving adsorption limits the extent to which kinetics
for Cu(I) reduction and the influence of Fe(II) on this reduction can
be quantitatively characterized. It is suggested that future work be
focused on developing such a model.
Conclusions
The overall currents measured in DC and AC voltammetry in
industry-emulating concentrations are the combination of currents
from Cu(II) reduction to Cu(I), and Cu(I) reduction to Cu metal. In
conditions designed to slow charge transfer kinetics, these two reduction steps can be separately observed by ACV. The shapes of the
harmonic current peaks for Cu(I) reduction support previous reports
that Cu(I) reduction occurs as an adsorbed species. In strongly acidic
conditions, Fe(II) lowers the overall reaction kinetics measured in
D23
DC and AC voltammetry. ACV results show that Fe(II) has a stronger
effect on Cu(I) reduction, producing lower peak currents at larger overpotentials. According to electrocrystallization theory, such limitation
of the deposition rate for Cu in presence of Fe (II) may be responsible
for the morphological features of the deposit growth reported in the
past.
Acknowledgments
The author acknowledge the MISTI-Chile program for its financial
support throughout this project.
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